The present invention relates to a fixed slit type photoelectric microscope for detecting a position of a linear pattern on an object.
Various types of photoelectric microscopes for optically detecting a position of a linear pattern have been proposed. A vibration type photoelectric microscope is disclosed in "Photoelectric Microscope" by S. Sasayama, Nikon Tech. J., No. 2, pp. 24-32, and in Japanese Patent Publication No. 52-46694 by S. Sasayama. In this photoelectric microscope, a luminous flux transmitted through a slit is modulated by mechanically vibrating a slit on which a linear pattern (scale lines) disposed on an object under inspection is formed. The modulated luminous flux is received and converted into an electrical signal by a photoelectric conversion element. This electrical signal is amplified by a preamplifier and rectified by a synchronous rectifier. The rectifying signal is displayed by an indicating instrument as a displacement of the linear pattern on the object under inspection. In the photoelectric microscope, since the luminous flux received by a photoelectric conversion element is modulated, use of an AC amplifier is possible. Accordingly, an SN ratio of the detected output can be improved.
In a conventional photoelectric microscope in which the slit is mechanically vibrated, the center of the vibration becomes displaced over a period of time. Therefore, the detection accuracy is limited within the range of this displacement.
An apparatus designed to overcome this disadvantage is disclosed in "Double Diffraction Gratings As Keys for High Contrast Alignment on Wafer Steppers", by G. Dubroeucq et al., Proc. of International Conf. on Microlithography, Microcircuit Engineering 1980, pp. 181-190. This device detects a displacement of an alignment mark of double diffraction gratings on a wafer with respect to the center position of a multi-slit on a reticle mask. A luminous flux incident on a photoelectric conversion element is modulated using an acoustooptic modulator. Therefore, it is possible to prevent the displacement of the center of the mechanical vibration. However, it is difficult to adjust the apparatus since it employs a double diffraction for the detection of the alignment mark. In other words, in this apparatus, the pitch of the multi-slit must be exactly coincident with a value which is the product of the pitch of the alignment mark and a lens magnification. Further, it is difficult to form both the alignment mark and the multi-slit.